The present invention relates to a gap control method and a gap control apparatus suitably used in a light exposure apparatus, in particular an X-ray exposure apparatus, used in the course of production of semiconductor devices.
With a recent demand for higher density and higher operating speed of a large scale integrated circuit (LSI), the miniaturization of elements is required. Finer element s can be formed as the wavelength of light used in photolithography in production of such an LSI, is shorter. In this connection, there is used an X-ray lithography using X-rays as a light source. In mask alignment in such an X-ray lithography, an X-ray mask and a wafer are disposed opposite to each other with a proximity gap of, for example, 20 .mu.m, provided therebetween, and X-rays are irradiated to the wafer such that predetermined parts thereof are exposed to the X-rays. By reducing the gap between the X-ray mask and the wafer, it is expected to produce a finer element. It is therefore required to set, with high precision, the gap between the X-ray mask and the wafer.
As the gap setting method in conventional mask alignment, there is used a method using a lens of double focal points or a lens of single focal point.
In FIG. 7 showing the method using a lens of double focal points, there are shown a lens of double focal points 51, a wafer 52, a wafer mark 53, an X-ray mask 54 and a mask mark 55. First, a first focal point of the lens of double focal points 51 is aligned with the wafer mark 53 on the wafer 52, and a second focal point is then aligned with the mask mark 55 on the X-ray mask 54, thus setting the gap between the wafer 52 and the X-ray mask 54.
FIG. 8 shows another gap setting method, i.e., the method using a lens of single focal point. In FIG. 8, there are shown a lens of single focal point 61, a lens driving system 62 and a drive-amount detecting system 63. First, the focal point of the lens of single focal point 61 is adjusted with a wafer mark 53 on a wafer 52. Then, the lens 61 is driven by the lens driving system 62 such that the focal point of the lens 61 is adjusted with a mask mark 55 on an X-ray mask 54. This drive amount is detected by the drive-amount detecting system 63, thus calculating the gap between the X-ray mask 54 and the wafer 52.
In an X-ray exposure apparatus, the gap between the X-ray mask and the wafer is set in the vicinity of 20 .mu.m, and mask alignment and X-ray exposure are then carried out.
On the other hand, there is known, as a high-precision mask-wafer alignment apparatus, an apparatus of the heterodyne system using so-called heterodyne interference light in which two laser lights respectively having slightly different frequencies, interfere with each other, as disclosed in a publication "Light Technology Contact" (vol. 28, No. 7, 1991, p. 389).
With reference to FIGS. 9 and 10, the following description will discuss the alignment principle in the heterodyne system.
As shown in FIG. 9, when two laser lights respectively having slightly different frequencies f1, f2 are incident upon a diffraction grating b in the respective directions of primary diffraction angles .theta.a, .theta.b which are symmetric with respect to a normal line of the diffraction grating b, the laser lights having the frequencies f1, f2 are primarily diffracted in the normal line of the diffraction grating b and interfere with each other to form a beat signal Ir( t ) . The beat signal Ir(t) is expressed by the following equation: EQU Ir(t)=.vertline.E1(t)+E2(t).vertline..sup.2 =A1.sup.2 +A2.sup.2 A1A.sup.2 .multidot.cos (2.pi..vertline.f1-f2.vertline.t+.phi.1-.phi.2)
wherein A1, A2 are the amplitudes of the laser lights, .phi.1, .phi.2 are the initial phases thereof, and E1(t), E2(t) are the amplitude intensities thereof which are respectively expressed by the following equations: EQU E1(t)=A1.multidot.exp j(2.pi.f1t+.phi.1) EQU E2(t)=A2.multidot.exp j(2.pi.f2t+.phi.2)
When the diffraction grating b is moved from the position shown by a solid line in FIG. 9 to the position shown by a broken line therein (displacement .DELTA.x), changes in light path length of the two laser lights are equal to .DELTA.x sin .theta.a, .DELTA.x sin .theta.b, respectively. Accordingly, a beat signal Ib(t) is expressed by the following equation: EQU Ib(t)=A1.sup.2 +A2.sup.2 +A1A2.multidot.cos {2.pi..vertline.f1-f2.vertline.t+.phi.1-.phi.2+2.pi..DELTA.x/(p/2)}
wherein P is the grate pitch in the diffraction grating.
That is, the phase difference between the beat signals Ir(t) and Ib(t) , is linearly changed in a cycle of the displacement amount P/2 of the diffraction grating b.
The alignment apparatus of the heterodyne system utilizes the principle above-mentioned. As shown in FIG. 10, a first diffraction grating a is attached to a mask m, a second diffraction grating b is attached to a semiconductor wafer w, and the mask m and the wafer w are opposite to each other with a gap G provided therebetween. In such an arrangement, changes in light path length of two laser lights are equal to G(1+cos .theta.a), G(1+cos .theta.b), respectively. When .theta.a is equal to .theta.b, such changes are equal to each other and therefore cancelled by each other. This does not cause the beat signal to be changed in phase. More specifically, even though the gap G is changed, no influence is exerted to positional detection because of the transversely symmetric optical systems.
It is now supposed that the positional shift between the mask m and the wafer w is .DELTA.x. In this case, the positional shift .DELTA.x between the mask and the wafer is detected when a phase difference (.phi.2-.phi.1) is detected according to the following equation (1): EQU .phi.1-.phi.2=2.pi..DELTA.x/(P/2) (1)
At this time, the phase difference (.phi.2-.phi.1) can be detected with precision of about 1.degree. including errors generated in an electric signal processing. As a result, the mask m and the wafer w can be positionally aligned with each other with high precision.
In the mask-wafer positional alignment apparatus using the heterodyne interference system above-mentioned, it is possible to positionally align the mask with the wafer with high precision corresponding to the high resolution of an X-ray exposure apparatus. In the conventional gap setting method mentioned earlier, it is possible to set the gap of about 1 .mu.m or more. However, it is not possible to set the gap of not greater than the focal depth of a lens to be used, and it is difficult to set the focal depth to 1 .mu.m or less. It is therefore difficult to set the gap between the mask and the wafer with high precision required in the X-ray lithography. This disadvantageously fails to sufficiently utilize the improved precision of mask positional alignment.